4 The Dynamic Load Testing Method Research program started in Case Institute of Technology (now Case Western Reserve University), with the objective of developing an economical, practical for field use, portable pile bearing capacity measurement system Method is based on electronic measurements of pile top force and velocity during impact of a large hammer Two kinds of sensors are installed on a location usually close to the pile top: Strain transducers for measuring the force (by multiplying the strain by the elastic modulus and crosssection area of the pile material) For piles up to about 6 diameter => use strain gages glued to rod For larger piles => use standard reusable gages Accelerometers for measuring the velocity (by integrating the acceleration data)

5 The Dynamic Load Testing Method Signals from sensors are sent via cable or radio to the Pile Driving Analyzer (PDA) After each blow the PDA processes the data, providing: Pile capacity using the simplified CASE method (valid for uniform piles) Maximum stresses along the pile Transferred energy The data collected for one blow can be further analyzed using the CAPWAP program, to determine: Total mobilized resistance Resistance distribution Simulated load-displacement curve

6 The CAPWAP Analysis CAPWAP is a Signal Matching Program (System Identification Analysis or Reverse Analysis) Load System Movement

7 The CAPWAP Analysis The pile is divided in elements roughly 3.3 ft. long => non-uniform piles can be easily modeled The soil is divided in elements usually about 6.6 ft long, plus an additional element for the toe A model based on the work of E.A.L. Smith (1960), with numerous extensions and improvements, is used for the soil One variable is used as input (for example velocity) and the soil model parameters are interactively changed until the best possible match between measured and the calculated complementary curves (for example force) is achieved The static resistance from this soil model is the mobilized resistance (can correspond to the ultimate resistance if sufficient energy was applied to cause substantial permanent displacement) The soil and pile model are used to generate a simulated load-displacement curve

11 Interpretation of CAPWAP loaddisplacement curve For small allowable displacements (e.g. Davisson), analysis of blow with highest mobilized capacity usually sufficient For larger displacements, superposition of load-displacement curves for all applied blows is recommended

14 J.G. Cannon (2000) Impact System: Special cable drop hammer made up by one of the Contractors Drop weights consist of 2 and 4 tonne (4.4 and 8.8 kips) solid circular billets of steel of about 350 mm (13.8 inches) diameter and 2.6 and 5 m (8.5 and 16.4 ft) long respectively Guide frame allows for a stroke of about 2 m (6.6 ft) The frame was supported laterally by 4 guy wires tied to adjacent screw piles, either production piles or temporary anchors specifically for the test

17 J.G. Cannon (2000) Redlands Mater Hospital, Brisbane, Australia 89x5.5 mm (3.5x7/32 inches) with a single 350 mm (13.8 inches) helix 114x6 mm (4.5x15/64 inches) with a single 450 mm (17.7 inches) helix Penetrations from 2.2 to 5 m (7.2 to 16.4 ft) Stiff clays over stiff to hard gravely clays 8 piles were tested, comprising 15% of total piles One of the test piles showed high torque resistance during installation, but during testing the set was very high (37 mm/bl 8 bl/ft) and the mobilized resistance was also low. According to the author, this is an example of why installation torque does not give a good indication of pile capacity.

18 J.W. Beim & S.C. Luna (2012) Test program performed on helical piles specially installed at the National Geotechnical Experimentation Site of the University of Massachusetts - Amherst campus (UMass-Amherst) Soil consists of approximately 5 ft of stiff silty-clay fill overlaying a thick 100-ft deposit of late Pleistocene lacustrine varved clay Seven piles were submitted to Dynamic Load Tests and also to Static Load Tests Piles were 2 7/8x0.217 inch, with 3 helices (8, 10 and 12 inches) Three piles were installed to a depth of 12 ft, and consisted of one bottom section and one extension. Five piles were installed to a depth of 18 ft and consisted of one bottom section and two extensions. The shorter piles showed more resistance than the longer ones, as expected due to a drop in soil resistance at 13 ft

22 A. Klesney & F. Rausche (2012) Ten 2-7/8 inch piles were tested at the Amtrak Passenger Station of Alpine, Texas Piles installed into primarily granular soil to a minimum depth of 10 ft The impact system consisted of a simple, 1,500 lb drop hammer with a reusable, portable pile extension fitted for transducer attachment and drop hammer impact. Three or four impacts with fall heights up to 3 ft were applied

23 B. White et al (2013) Four 9 5/8x0.395 inch helical piles tested in Midland, Michigan. The piles had two helices of the same diameter: 20 (2), 22 and 24 inches. The design pile capacities were 87, 96 and 111 kips, respectively. Piles penetrated ft in the soil described as compact loamy sand to 9 ft depth, stiff clay extending to 45 ft depth, over an extremely hard layer of clay below 45 ft depth. Blows were applied by the APPLE VI dynamic load testing system, consisting of a 4.5 ton drop weight supported by a metallic frame. The drop heights ranged from 2 to 18 inches Load-displacement curves were constructed by superimposing the CAPWAP load-displacement curves of the individual blows. Capacities were evaluated for a ¼ inch and for a ½ inch top displacement (the latter was the specified performance criterion)

Step 11 Static Load Testing Test loading is the most definitive method of determining load capacity of a pile. Testing a pile to failure provides valuable information to the design engineer and is recommended

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